| Vertical Blanking Interval |
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The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT ; the magnetic field, and hence the position of the spot on the screen, cannot change instantly. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the ''horizontal retrace'' or ''horizontal blanking interval''. While modern equipment does not require a long blanking time, the standards were drawn up to cope with older equipment. In analog television systems the vertical blanking interval can be used to carry digital data, since nothing sent during the VBI is displayed on the screen; various test signals, Time Code s, Closed Captioning , Teletext , CGMS-A copy-protection indicators, and various data encoded by the XDS protocol (e.g., the Content Rating s for V-chip use) and other digital data can be sent during this time period. The pause between sending video data is used in real time computer graphics to perform various operations on the Back Buffer before copying it to the front buffer instead of just switching both Pointer s, or to provide a time reference for when switching such pointers is safe. In video game systems the vertical blanking pulses are extensively used for timing, as they occur at an accurately known frequency. Most graphics operations on consoles up to and including the 16-bit Era could be performed only during the VBI (which programmers generally referred to as the VBLANK), requiring programs to do all graphics processing rigidly within it. The need to synchronise game code this way made early video game systems such as the Atari 2600 difficult to program. Most consumer VCR s use the known black level of the vertical blanking pulse to set their recording levels. The Macrovision Copy Protection scheme inserts pulses in the VBI, where the recorder expects a constant level, on videotapes to disrupt recording. EXTERNAL LINKS |
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